1. Field of the Invention
The present invention is directed to the field of growing bulk semiconductor crystals from silicon melts by the Czochralski method. More particularly, this invention relates to a coating material for absorbing radiant heat emitted from the crystal, a manufacturing method thereof, a cooling apparatus coated with the same material, and a crystal growing system for producing silicon crystals using the apparatus enabling to improve pulling speed and cooling rates of the crystal by dissipating the radiant heat emitted from the crystal effectively outside so as to improve productivity and quality of the crystal.
2. Background of the Related Art
In order to cope with the increasing demands for semiconductor fabrication, quality requirements for a silicon wafer become continuously stricter.
When a silicon single crystal is grown by the Czochralski(CZ) method, the cooling rate of the crystal has great influence on formation of grown-in defects, Hence, in the process of growing the silicon crystal, productivity improvement and effective control of the defects are discriminating factors.
FIG.1 illustrates the diagrammatic path of heat flow in a crystal growing apparatus during the silicon crystal growth. In equilibrium state, heat balance quation during the silicon crystal growth can be expressed as follows.
Heat Balance: QR=QC=QL+QM, where QR, QC, QL and QM are the total amount of heat emitted from crystal surface per unit time, the amount of heat conducted by crystal per unit time, the amount of latent heat of crystallization per unit time, and the amount of heat transferred from melt to crystal per unit time, respectively.
Crystal pulling speed V=(KsGsAxe2x88x92KLGLA)/(▾SLA) =(QRxe2x88x92KLGLA) (▾SLA), where V, KS, GS, KL, GL, ▾, and SL are the crystal puling speed, thermal conductivity the crystal, the temperature gradient in the crystal near the crystallization interface, the density of the crystal, and latent heat of crystallization(per unit mass), respectively.
Therefore, the crystal pulling speed(V) depends exceedingly on the temperature gradients in the crystal and the melt.
The conventional technique has normally used a method of inserting heat shield between the melt and the crystal to increase the temperature gradient in the crystal near the interface of crystallization. Yet, the temperature gradient rise in the crystal by such a technique has a technical limitation.
The inner wall of the crystal growing furnace, which play a role of absorbing and dissipating radiant heat in the crystal growing system is made of a metal based material having low emissivity such as stainless steel, thereby failing to receive effectively radiant heat emitted from the crystal. Thus, the radiant heat is reflected to the crystal again, thereby having technical barrier in increasing the cooling rate of the crystal.
In order to increase the emissivity, the inner wall of the furnace should be formed of a material having high emissivity. However, such a material having high emissivity and satisfing strict requirements for application to the crystal growing furnace, has not been found yet. A method of coating on an inner wall of the crystal growing furnace with a material having high emissivity can be considered. However, such a coating material, which enables to overcome thermal stress caused by the difference of thermal expansion coefficient between a coated surface and a coating material in a wide temperature range from the room temperature to the high-temperature (xcx9c1,000 C.) and to adhere strongly to stainless steel, has not been found so far.
Therefore, the present invention is directed to a coating material for absorbing radiant heat, a forming method thereof, a radiant heat cooling apparatus using the same, and an apparatus for growing silicon single crystals using the same that substantially obviate one or more problems due to the limitations and disadvantages of the conventional art.
An object of the present invention is to provide a coating material enabling to adhere strongly to stainless steel in a wide temperature range from between the room temperature to the high-temperature region(xcx9c1,000 C.) as well as have high emissivity.
The coating material according to the present invention includes a silica matrix and graphite power having a high emissivity, and is economically applicable to mass production.
Another object of the present invention is to provide a method of forming a coating material enabling to adhere strongly to stainless steel in a wide temperature range from the room temperature to the high-temperature region(xcx9c1,000 C.) as well as have high emissivity.
A further object of the present invention is to provide an apparatus for growing silicon single crystals enabling to increase the productivity of crystal growing by cooling the crystal at a great rate by absorbing effectively radiant heat emitted from the crystal.
Another further object of the present invention is to provide a radiant heat cooling apparatus enabling to improve a crystal cooling rate by maximizing heat dissipation effect by means of absorbing effectively radiant heat emitted from the crystal
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages in accordance with the purpose of the invention, as embodied and described herein, a radiant heat absorbing coating material according to the present invention includes 10xcx9c15 wt/% of silica, 5xcx9c10 Wt/% of graphite power, and 75xcx9c85 wt/% of IPA(isopropyl alcohol).
Preferably, the IPA is volatilized by drying and particles of the graphite powder are disributed homogeneously in the silica matrix and attached to the stainless steel material together with silica.
In another aspect of the present invention, a method of forming a radiant heat absorbing coating material includes the step of agitating to mix 10xcx9c15 wt/% of silica gel, 5xcx9c10 Wt/% of graphite powder, and 75xcx9c85 wt/% of IPA(isopropyl alcohol).
In a further aspect of the present invention, in the crystal growing apparatus for growing silicon single crystals from the silicon melts, the silicon single crystal growing apparatus comprises a quartz crucible, a heating element, a crystal pulling device, a heat shield for maintaining a proper temperature distribution in the crystal growing system, and a cooling device, wherein the quartz crucible, heating element, crystal pulling device, heat shield, and cooling apparatus are installed inside a sealed furnace or a sealed crystal growing apparatus, wherein the silicon single crystal growing apparatus further comprises the crystal cooling apparatus for cooling the crystal during the crystal growth by absorbing radiant heat emitted from the crystal, and wherein the crystal cooling apparatus includes a circulating cooling pipe inside and a radiant heat absorbing layer coated on the surface of the cooling instrument.
Preferably, the cooling apparatus is made of stainless steel and wherein the radiant heat absorbing layer is formed by coating a radiant heat absorbing coating material on the surface of the instrument, which is prepared by agitating to mix 10xcx9c15 wt/% of silica gel, 5xcx9c10 Wt/% of graphite powder, and 75xcx9c85 wt/% of IPA, thereon, drying the coated radiant heat absorbing coating material, carrying out low-temperature sintering thereon.
In another further aspect of the present invention, in a cooling apparatus for absorbing radiant heat for transfering heat outside the crystal growing apparatus or furnace, a radiant heat cooling apparatus includes a circulating cooling pipe inside and a radiant heat absorbing layer coated on the surface of the cooling apparatus, wherein the radiant heat absorbing layer is formed by coating a radiant heat absorbing coating material, which is prepared by agitating to mix 10xcx9c15 wt/% of silica gel, 5xcx9c10 Wt/% of graphite powder, and 75xcx9c85 wt/% of IPA, thereon and drying the coating material.
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.